<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        console.log(a);   //undefined
        // console.log(b);  //报错
        var a;
        const b=2;
        const c=234;
        // c=123;
        // console.log(c);
        const d ={
            name:'tom'
        }
        // d ={
        //     name:'rose'
        // }
        d.age=13;
        console.log(d);  //name:'tom' age:13
    </script>
</body>
</html>